1. Field of the Invention
This invention is in the field of sandwich-type structural materials, and in particular relates to sandwich-type structural materials with acoustic attenuation characteristics.
2. Description of the Prior Art
Existing launch vehicle and aircraft structures are still overwhelmingly fabricated using aluminum and steel. These materials are often very heavy for this weight critical application by today's structural standards. These materials are utilized, though, due to their damage tolerance and low cost manufacturing techniques. Aerospace structures that are fabricated using advanced composite materials, while lighter weight, suffer from poor damage tolerance and very high manual labor requirements during fabrication.
An additional problem, specific to launch vehicles, is that a significant number of spacecraft failures can be attributed to the conditions experienced by the spacecraft during launch into orbit. These conditions include very high accelerations, severe vibration due to lift-off and staging and very high broadband acoustic levels, particularly during liftoff. Current launch vehicle systems require that spacecraft be designed to withstand these high acceleration and vibration levels. To protect the spacecraft in this extreme acoustic environment, every existing launch vehicle system incorporates very heavy acoustic blankets and distributed masses in the fairing surrounding the payload area of the vehicle.
The Boeing Company's "MacroCore" concept incorporates Helmholtz resonators to attenuate sound. Although a very good idea, this structural concept requires a wall thickness that is unrealistic for most launch vehicle applications. The Helmholtz resonators, in this concept, are a product of the sandwich core construction, and are, therefore, limited by the sandwich core concept. Specifically, this concept is susceptible to the weaknesses of core/facesheet delamination and moisture retention.
Leppington discusses the use of perforated facesheets to create Helmholtz resonators out of honeycomb core pockets (Leppington, F. G, K. H. Heron, E. G. Broadbent, and S. M. Mead, "The Acoustic Transmission Properties of Anisotropic Sandwich Panels With Perforations," Proc. R. Soc. Lond. A 428, 27-47, 1990). This structure is limited in that it is, similar to the MacroCore concept, basically a sandwich core construction. Therefore, it is limited in that it is susceptible to facesheet/core delamination and moisture incursion. Additionally, the structural requirements on the core material drives the resonator volume which, for any practical geometry, is targeted at a frequency range higher than that which is useful for launch vehicle applications.
U.S. Pat. No. 3,831,710 entitled "Sound Absorbing Panels" incorporated Helmholtz resonators into a sandwich core structural concept. This concept visualizes the incorporation of Helmholtz resonators into structures but doesn't describe how such structures could be constructed. The concept envisions "grid-stiffened" structures to create this geometry. "Grid-Stiffened" structures have not, to date, been fabricated in this form and are very difficult to fabricate in the geometry envisioned by the patent.